Technical Field
The instant disclosure relates to a fingerprint sensor and a method for manufacturing the fingerprint sensor, and particularly relates to a suspended capacitive fingerprint sensor and method for manufacturing the same.
Related Art
Along with the developments in semiconductor integrated circuits (IC) manufacturing processes, new capacitive fingerprint sensing chips reestablish the era for fingerprint recognition systems, and global chip manufacturers develop many press-type and slide-type fingerprint sensors gradually. Based on this new technology, electronic device manufacturers have made laptops having capacitive fingerprint sensor chips since 2000, like IBM and Dell. Also, from 2006, Asus and Acer started to sell laptops having the capacitive fingerprint sensors. However, the fingerprint recognition function is mainly utilized in business laptops and provided for replacing the boot password and for encrypting files. In addition, since the capacitive fingerprint sensors are still lack of their own specific applications, installation rates of the capacitive fingerprint sensors in laptops are hardly to be increased.
As technologies move forward, more and more electronic devices appear in our daily lives. In addition, the high speed internet century promotes the growing in mobile payment and electrical commerce, and online transactions become frequently. Therefore the importance of electronic authentication is increasing. In the existing password authentication, the passwords may be stolen, forgotten, or cracked, and users are required to change the passwords regularly. Such complicated requirements in identification (ID) and password may not only lower the willingness of customers in online transactions, but also increase risks and costs for the password authentication service providers. Consequently, to replace the conventional password authentication with a convenient and safe personal ID authentication would improve the willingness and confidence of customers in online transactions and would be positive to the developments of the mobile payment and electrical commerce.
As compared to a conventional optical recognition device, a capacitive fingerprint recognition chip is small and has the ability for identifying living things. Therefore, the capacitive fingerprint recognition chip would not be cheated by a fake finger easily. In addition, because the application of the capacitive fingerprint chips is no more restricted in governments' recognition systems, the application of fingerprint recognition in consumer electronics grows gradually in the last decade; consumer electronics including flash disks, portable hard drives, laptops, and mobile phones may have fingerprint recognition chips to protect user information. After Apple launched its iPhone 5s installed with fingerprint recognition chip in September of 2013, other mobile phone companies began to launch their devices installed with fingerprint recognition functions. Accordingly, applications of fingerprint recognition in consumer electronics became popular again. In addition, because of the rapid eruption of electrical commerce and mobile payment, fingerprint recognition chips may possibly be the standard equipments of mobile devices.
Fingerprint recognition devices are the main stream in biological recognition technologies. The conventional optical fingerprint reading methods, as disclosed in U.S. Pat. No. 4,053,228 and U.S. Pat. No. 4,340,300, are maturely developed; however, the size of the device is large and the cost of the device is not cheap.
Moreover, as disclosed in CN patent 1228731, where the inventors utilize the pressure sensor as the fingerprint sensing unit. In brief, the capacitance change between the suspended substrates is induced by using a user's finger to press the protruded blocks of the suspended substrates, so that the fingerprint of the user's finger can be recognized. However, since a human's finger is soft and the structure rigidness of a suspended substrate, with its area not exceeding than 50*50 micrometer square (μm2), is strong, the capacitance change between the suspended substrates is quite small when the user's finger presses on such suspended substrate in a 500 dpi fingerprint resolution requirement. In addition, during operation, the finger has to directly in contact with the suspended substrate, so that the suspended substrate may be damaged easily. Moreover, the suspended substrates are not suitable in nowadays chip packaging procedures of portable electronic devices.
Because of the popularization of portable electronic devices, attentions are paid to small and light fingerprint sensors gradually, promoting the developments of capacitive fingerprint sensors. And accordingly, silicon chips are served as the fingerprint recognition chips as disclosed in U.S. Pat. No. 7,864,992 and U.S. Pat. No. 6,512,381. For a conventional sensor unit, external or internal driving signals may be applied thereto to generate signals. Specifically, a group of electrode plates made by semiconductor manufacturing processes are provided to be in contact with a user's hand, an insulation layer is covered on the electrode plates, and the electrode plates may be perpendicularly or parallel aligned with each other. Because human bodies are conductive, the finger and the electrode plates form a sensing capacitance when the finger is in contact with the electrode plates. The sensing capacitance value formed by the ridge of the finger and the electrode plates is different from that formed by the valley of the finger and the electrode plates; specifically, the capacitance value formed by the ridge and the electrode plates is greater than that formed by the valley and the electrode plates. Accordingly, the sensing capacitance is compared with a reference capacitance and a parasitic capacitance, so that a two dimensional graph of the fingerprint showing ridges and valleys can be obtained. However, the sensing capacitance value would be affected by the protection layer, i.e., the thicker the protection layer is, the smaller the sensing capacitance value becomes. In addition, if the difference between the sensing capacitance value and the reference capacitance value is too large, the signal of the fingerprint sensing unit outputted to the backend processing circuit would be reduced, lowering the sensitivity of the sensor unit.
For the fingerprint sensor application in laptops, because of the book-like appearance of the laptop, the fingerprint sensor would be protected by the cover of the laptop, allowing the thickness of the protection layer of the fingerprint sensor to be in the range of 5 to 10 μm. While for portable electronic devices, the fingerprint recognition chip is to be exposed to outside for meeting users' needs and mating with the structure of the device, consequently, in the portable electronic device, the thickness of the protection layer covering on the fingerprint recognition chip is to be increased. In conventional semiconductor manufacturing processes, the reference capacitance is formed by an insulation layer commonly made of silicon dioxide. The minimum value of the reference capacitance is Cref=∈*A/n*d, wherein ∈ is the dielectric constant, A is the area of the sensing electrode, D is the thickness of the silicon dioxide layer for each layer, n is the number of the layers of the silicon dioxide layers, and the dielectric constant of the insulation layer is approximately equal to 4.
For the conventional fingerprint recognition chip, no matter the driving signal is provided by an external driving source or an internal driving source, the thickness of the protection layer, the dielectric constant of the protection layer, and the intensity of the driving signals are positively related with each other. In order to utilize thicker materials as the protection layer, some companies use a sapphire substrate as the protection layer since the dielectric constant of sapphires is about 9 to 11; while other some companies use a modified molding compound with its dielectric constant about 7 to 8 to replace the conventional molding compound. However, the cost of the sapphire substrate is expensive; in addition, the high dielectric constant molding compound is not stable, thus the defect-free rate in packaging procedures of the fingerprint would be affected adversely and easily.